Switched capacitor filters are known. Such filters are the product of filter designing techniques to miniaturize filters, which are often reduced to integrated circuit (IC) form. A switch capacitor filter utilizes the fact that when a capacitor is switched between a signal to be sampled and ground at a frequency many times the frequency of the sampled signal, the capacitor will simulate the circuit behavior of a resistor. In the design of switched capacitor filters, summing amplifiers are frequently employed as an input section to select one or more signals to be filtered by the switch capacitor filter. Such a summing amplifier is illustrated in FIG. 1. The switch capacitor summing amplifier 100 is comprised of a plurality of input circuits 102 and a conventional switch capacitor amplifier 104. Each input section is coupled to amplifier 104, which sums any of the input voltage signals that may be present (V.sub.1 -V.sub.n) and provides an output signal 114. Each of the input sections 102 is comprised of two MOS transistor switches 106 and a capacitor 108. The amplifier 104 is comprised of an operational amplifier 116 having feedback capacitors 110 and 112, and has a pair of sampling switches (106) at the input and output. The OP amp (116) provides an output signal 114, which may be advantageously filtered by any suitable switched capacitor filter known in the art. As is readily understood in the art, there are commonly two phases of sampling associated with the switches 106. These are an even phase and an odd phase. Typically, these signals are of complementary phase and are easily generated using an inverter or functional equivalent. All the switches marked "even" are closed simultaneously, then opened followed by a closure of all the switches marked "odd".
Frequently, the summing amplifier 100 operates as a signal selector and often has all but one of its signal sources disabled. However, the continued connection of the remaining unused inputs seriously impact performance of the summing amplifier 100. If the unused inputs are allowed to "float", parasitic charge may couple across the capacitor 108 and cause shifts in the frequency response of the summing amplifier 100. Conversely, if the unused inputs are connected to an AC ground, the input noise, DC offset and switch impedance sensitivity of the amplifier 104 are severely degraded. Thus, there is a need in the art to provide a switch capacitor amplifier that operates to select various input signals without degrading the performance of the following switch capacitor filter.